I'd go with Montserrat as well, but keep in mind it's pretty limited when it comes to different font weights from Google Fonts. I usually play it safe and use Raleway as it comes with so many weights. Only downside is no italics - but who needs em?! https://www.google.com/fonts/specimen/Raleway

We use this font on one of my biggest clients website in regards to UX as it is so legible and pairs very well with many Serif fonts. Try looking into Domine, Poly, or Enriqueta as great pairs. All on Google Fonts as well.